Text by Leila Pazouki

Some months after we published the original research article entitled “Germacrene A Synthase in Yarrow (Achillea millefolium) Is an Enzyme with Mixed Substrate Specificity: Gene Cloning, Functional Characterization and Expression Analysis” in Frontiers in Plant Science journal (see the blog post here), they invited us to write Focused Review article based on the recommendation of Prof Kazuki Saito, the chief editor of the specialty section. The original research article was democratically chosen based on the Frontiers online analytics systems to “climb the tier”.

The idea was to put this work into a broader context, explain what the main questions are and where this work might lead.

As the main focus of the article (Germacrene A Synthase in Yarrow…) was the isolation of multi-substrate terpene synthase, we focused in this review paper on the occurrence, physiological significance and proposed process for the evolution of multi-substrate terpene synthases in plants.

Our study suggests that multi-substrate use is more common in plants than generally thought and advocates for conduction of further systematic studies using multiple substrates across phylogenetically different plant groups harboring terpene synthases (TPSs) from different clades to gain an insight into the existence of the capacity for multi-substrate use across plant kingdom. The overall significance of alternative activities of multi-substrate enzymes will critically depend on the enzyme specificity and relative availability for different substrates. Perturbation of terpenoid metabolism under stress conditions can lead to enhanced substrate exchange between cytosol and plastids as well as modifications in the expression of enzymes responsible for product pool sizes and thus, favor synthesis of terpenoids according to non-conventional pathways.

Terpene synthases are responsible for synthesis of a large number of terpenes in plants using substrates provided by two distinct metabolic pathways, the mevalonate-dependent pathway that is located in cytosol and has been suggested to be responsible for synthesis of sesquiterpenes (C15), and 2-C-methyl-D-erythritol-4-phosphate pathway located in plastids and suggested to be responsible for the synthesis of hemi- (C5), mono- (C10), and diterpenes (C20). Recent advances in characterization of genes and enzymes responsible for substrate and end product biosynthesis as well as efforts in metabolic engineering have demonstrated existence of a number of multi-substrate terpene synthases. This review summarizes the progress in the characterization of such multi-substrate terpene synthases and suggests that the presence of multi-substrate use might have been significantly underestimated. Multi-substrate use could lead to important changes in terpene product profiles upon substrate profile changes under perturbation of metabolism in stressed plants as well as under certain developmental stages. We therefore argue that multi-substrate use can be significant under physiological conditions and can result in complicate modifications in terpene profiles.